1. Field of the Invention
The present invention relates to a technique for estimating the errors of the orthogonal modulator and the orthogonal demodulator without switching a path for an RF signal.
2. Description of the Related Art
A structure of a DPD (Digital Pre-Distorter: Digital Nonlinear Distortion Compensator) using the orthogonal modulator/demodulator has been known widely and generally. For example, in FIG. 3 of JP-3198864-B2, a portion shown on a left of a D/A converter 308 and an A/D converter 322 corresponds to the structure.
Although the DPD can compensate a nonlinearity, however, an amplitude error, a phase error (orthogonal error) and a DC offset in the modulation and demodulation of I/Q signals in the orthogonal modulator and the orthogonal demodulator remain. Moreover, these errors which remain also influence a compensating performance of the nonlinearity of the DVD. For this reason, it is necessary to remove them separately.
On the other hand, a structure described in FIGS. 1 and 2 of JP-2000-270037-A has been known as a technique for compensating the error in the orthogonal modulator, for example. Moreover, a structure described in each of the drawings of JP-2003-309615A has been known as a technique for compensating the error in the orthogonal demodulator, for example.
All of the techniques have such a structure as to include a feedback system for demodulating outputs of orthogonal modulating/demodulating portions to detect an error.
Also in a structure in which the feedback system in the technique described in JP-3198864-B2, and the feedback system in the technique described in JP-2000-270037-A or the technique described in JP-2003-309615-A are simply shared, there is no means for deciding whether the error is made by the orthogonal modulator or the orthogonal demodulator or is made as a nonlinear error. For this reason, it is necessary to individually provide the feedback systems in the orthogonal modulator and the orthogonal demodulator. Therefore, a circuit scale is to be increased.